The term "channel hopping" is used in this document as a collective designation for hops between different information transmission channels, for instance such as hops solely between frequencies, hops solely between time slots, and hops between both frequencies and time slots in a radio communications system.
The person skilled in this art is well aware of the fact that frequency hopping can be used in a radio communications system to improve the performance of the radio system or to guard against unauthorized listening of the radio communication, among other things. Frequency hopping is carried out in a predetermined order in such systems, without paying attention to the instant quality of the connection. Frequency hopping in radio communications systems is thus not adapted.
There can be established between a transmitter and a receiver of a radio communications system a radio connection over which radio communication can take place. The connection is bidirectional by virtue of a downlink which forms the connection in a direction from a base station in the system to a mobile station, and an uplink which forms the connection in the opposite direction, from the mobile station to the base station. The transmission and reception of radio traffic for different connections is effected on channels which can be defined by a certain frequency in an FDMA system or by a combination of a certain frequency and a certain time slot in a system which uses TDMA (Time Division Multiple Access). In a CDMA system, a channel can be defined by a code. Seen generally, the channels that are available in a radio communications system are liable to be disturbed by other radio traffic to different degrees of disturbance, and also by radio signals on the same channels used for other connections, each channel in the system having a certain interference level. Thus, if each connection uses only one channel, the connections will suffer different levels of interference. The levels of interference in certain connections may be so high as to prevent an acceptable connection quality being obtained. These disparities in connection quality can be levelled out by hopping between different channels, wherewith the connections utilize both channels of low and high interference levels. The use of the high interference channels is therewith spread out between the various connections, and when considering the system as a whole more connections can be given an acceptable quality with the aid of interleaving and error correction coding. Each connection can be allocated a plurality of channels, wherein the system controls the connections as communication proceeds, by causing the connections to hop between the channels in accordance with a given hop rule. This rule may be, for instance, a predetermined pseudo-random series, in which case the connections hop seemingly at random between all available channels; see for instance European Patent Application EP 93905701-4. However, the level of interference may become unnecessarily high when this type of channel hopping is used, since the channels are not always allocated to the connections in an optimal fashion when using a pseudo-random series.
A radio communications system will normally include a number of channels that can be used for connections between a given base station and mobile stations. In this case, it is important that the same channels are not used at the same time for two or more connections between the base station and mobile stations. If two transmitters transmit different signals to their respective receivers simultaneously on one and the same channel, it is very likely that at least one receiver will be disturbed by interference from the transmission to the other receiver. If the aforesaid situation cannot occur, i.e. when only one base station connection can be transmitted on a channel at a time on any one occasion, there is obtained what is known as "orthogonality in the base station" or "base orthogonality".
When a connection in a radio communication system is excessively bad, wherewith no acceptable connection quality is obtained, this may be due to the fact that the ratio between signal strength and interference is too low, among other things. The signal strength referred to in this regard is the strength of the received desired signal. By interference is meant the sum of the signal strengths of all received undesirable signals on the channel used. These undesirable signals arrive primarily from other connections that use the same channel in adjacent cells in the radio communications system. The received undesired signals may also originate from connections within the own cell, or local cell, these connections using a bordering frequency or time slot.
Signal strength is contingent on transmission power and also on the extent to which the desired signal has been attenuated on its way from the transmitter to the receiver. Signal attenuation is determined, among other things, by the distance, the direction and the topology between transmitter and receiver. Other terms used in parallel with attenuation are channel amplification or path gain (channel amplification is negative) and path loss, which are terms well-known to the person skilled in this art.
Several proposals dealing with different frequency of methods are well-known to the art. The following examples of known techniques use frequency hops in different ways to achieve particular objects in different types of communication systems.
German published specification DE 3415032-A1 teaches a frequency hop system in which frequency hopping is effected in a pseudo-random fashion. The frequencies used are monitored and excluded from further use when they no longer have an acceptable interference level.
U.S. Pat. No. 4,998,290 describes a radio communications system which uses frequency hopping. The system includes a central checking station which allocates frequencies for communication with several participant local radio stations. The checking station maintains an interference matrix which reflects the capacity requirement of the different radio stations and the interference state of all connections.
U.K. Patent Application GB 2,261,141 A teaches a method of using frequency hopping in a radio communications system. The method involves monitoring the channels included in the hop sequence and replacing those channels which do not fulfil a quality criterion with new channels.
U.S. Pat. No. 4,872,205 describes a frequency hopping communications system. The system detects automatically when another radio system is within range and then selects another group of frequency hop sequences, with the intention of avoiding mutual interference between systems whose ranges overlap one another.
U.S. Pat. No.5,210,771 describes a communications system in which each channel is allotted a desired limit value for received signal strength. A channel is allocated to a subscriber in dependence on the received signal strength at a receiving location and by the power control range of the subscriber unit. According to this patent specification, it is desirable to adjust the power of all users dynamically, in a manner such that signals will be received with approximately the same power.
U.S. Pat. No. 4,670,906 describes a method for the dynamic selection of one of a plurality of radio transmitters for transmitting message signals from a primary station to remote stations in a data communications system. The method involves measuring the signal strength of the carrier wave signal received by base stations with each transmission from an indicated remote station, calculating path loss between the indicated remote station and each base station location, while using the measured signal strength for the receiver at this location, calculating the signal strength that can be received at the indicated remote station from each base station, and selecting at least one of the base station transmitters for transmission of a message signal to the indicated remote station.
U.K. Patent Application GB 2,203,314 A describes a frequency hop allocation system for frequency hopping radio stations. According to one object of the invention described in this application, hop data can be allocated to disparate networks, so as to reduce interference between the networks to a level which will not prevent the radio stations from communicating with one another.
U.S. Pat. No. 4,355,399 describes a transmitter and receiver arrangement which enables the performance of an associated user to be controlled by selective, simultaneous transmission of one or more coded frequencies with each time slot of the transmitted sequence, wherein a higher degree of system flexibility is obtained and/or the performance of users with the worst reception conditions can be improved at the cost of those users which have better reception conditions, which results in improved system effectiveness as seen in total.